The congestion charge is another factor in the falling income: money made from that has gone down by £6m, because more cars in central London are eco-friendly and are thus exempt from the charge.

That puts Sadiq Khan's fare freeze plan in a bit of a tight spot - it's already forecast to cost Transport for London £640m.

Val Shawcross , Deputy Mayor for Transport at City Hall, told LBC:

“We are delivering major efficiency savings that have already reduced the cost of operations by £129m compared with the first three quarters of 2015/16 – far outweighing this slightly lower fare income – and generating more money from our commercial development.”